In fibre-optic endoscopes used in laparoscopy, a lens focuses an image of the object on the distal ends of a coherent bundle of optical imaging fibres. The image formed at the proximal end of the optical imaging fibres can be formed by a suitable lens into a real image for direct viewing, or can be focussed onto the image sensor of a video camera. The imaging bundle is surrounded by a layer of illuminating fibres through which light from a suitable high-intensity source is conducted to the distal end of the endoscope to illuminate the object.
Known video-based fibre-optic imaging systems are usually assembled from standard, commercially-available components: the image from the imaging bundle is focussed on the image sensor of a color video camera, and the resulting video signal is displayed on a commercial color video monitor. The illuminating fibres are normally illuminated with light generated by a 300-Watt Xenon-arc, a 150-300-Watt metal halide light source, or some other suitable light source. Video cameras used in known video-based imaging systems use systems developed for the consumer and industrial video markets to control parameters affecting image quality.
Most currently-available video-based fibre-optic imaging systems are optimized for large-diameter endoscopes having an outside diameter in the range of 5 to 10 mm (0.2" to 0.4" ) and using standard rod lens assemblies. Endoscopes having a considerably smaller outside diameter in the range of 1 to 2 mm (0.04" to 0.08" ) using Gradient Index (GRIN) lenses and fibre-optic imaging bundles have been developed and are also available for surgical applications. Such endoscopes are advantageous in that they further reduce the size of incision required to insert them into a body cavity.
While some known video imaging systems are capable of generating images from small diameter endoscopes, they are typically restricted to use at short working distances, typically less than 2" (50 mm). If the image from the fibre-optic assembly is formed on the image sensor in the camera so that the image covers the whole area of the sensor, the resulting video picture of an object at an extended working distance has insufficient intensity when normal levels of illumination are used. Moreover, the video picture of an object at any working distance is pixellated, i.e., the picture clearly shows the outlines of the individual optical fibres of the imaging bundle and the voids around them, if present. These shortcomings are a result of the small diameter of the imaging bundle, and the relatively few (typically 1,600 to 25,000) optical fibres in the imaging bundle of a small-diameter endoscope.
A more acceptable video picture is obtained by reducing the size of the image of the imaging bundle formed on the image sensor in the camera so that the image occupies a fraction of the area of the sensor. This arrangement produces a video frame in which a central image of the imaging bundle is surrounded by a blank external area, and results in a video picture in which the intensity of the image is increased and the pixellation of the image is reduced. However, this arrangement also has some disadvantages. The pixels of the image sensor in the external area surrounding the image generate noise, especially when the light level of the image is low. This noise is visible in the blank external area of the frame, and can be distracting to the observer.
Small-diameter fibre-optic endoscopes present additional problems when used in large body cavities. In such applications, endoscopes with a hyper-extended working distance, greater than 50 mm (2" ), are used. With such an arrangement, the light level on the sensor in the video camera is low, which exacerbates the noise problem in the external area surrounding the image.
In known video display systems for small-diameter endoscopes, the location of the image in the frame displayed on the monitor corresponds to the location of the image on the image sensor in the camera. Thus, the image is nominally displayed in the center of the monitor screen, but the position of the image on the sensor, and hence on the screen, is not accurately determined due to mechanical tolerances in the optical assembly. This layout requires the use of external equipment, such as a video switcher, to be able to display auxiliary information, such as patient monitoring data, or video system parameters, in the external area outside the image, or to be able to display multiple endoscope images on the same monitor. However, satisfactory results are not always obtained because of tolerance in the position of the image on the sensor.
Additionally, displaying the image in the same place on the monitor screen can, over time, cause a sharply-delineated burn area on the screen. The boundary of the burn area becomes noticeable if the diameter of the image increases, or if the position of the image on the screen changes. Screen burn is exacerbated if the image is always displayed in the same position on the screen.
It is known in consumer video systems to derive a signal for operating an auto focus system from an small area, normally in the center, of the image sensor in the camera. It is also known in endoscopic video systems to derive a signal for adjusting white balance from a small area, normally in the center, of the image sensor in the camera. In both of these known systems, however, the relationship between the small area and the image is undefined.